Engineering topics

Willson, R. C.

Publications and source records attributed to Willson, R. C..

At least 37 records · Page 2

NASA NTRS1982

Latest rocket measurements of the solar constant

Three rocket flights which carried a payload of absolute radiometers to measure the solar constant with an accuracy of plus or minus 0.5 per cent have been accomplished. Several of the rocket radiometers were duplicates of those aboard the Solar Maximum Mission and Nimbus spacecrafts. The values for the solar constant obtained by the rocket sensors for the three flight dates indicate an increase between the first and latter two flights approximately equivalent to the uncertainty of the measurements. The values for the solar constant for the three flights are 1367, 1372 and 1374 W/sq m.

Duncan, C. H.↗

NASA NTRS1981

Active cavity radiometer

The active cavity radiometer (ACR) experiment on the Spacelab 1 mission to measure the total solar irradiance is discussed. Short and long term variations in the total solar output of optical energy are studied. Solar total irradiance observation provides information on the solar cycle and other long term trends in solar output that are of climatological significance as well as short term solar physics phenomena. The interaction of solar radiation with the Earth's atmosphere, oceans, and land masses provides the primary driving forces for the formation of weather systems and the determination of climate. Astrophysical measurements determine the total energy flux. The principal role of the ACR observations support extended solar irradiance experiments on free flying satellites. Solar irradiance measurements are important in the establishment of the radiation scale at the solar total flux level in the international system of units (SI).

Willson, R. C.↗

NASA NTRS1981

Solar total irradiance observations by active cavity radiometers

Investigations have been conducted concerning the development of new cavity pyrheliometer for an absolute measurement of solar irradiance both in preflight testing of spacecraft and in flight experiments to measure the total solar flux. A series of instruments developed as part of this program are electrically self-calibrated cavity pyrheliometers whose mode of operation is characterized by the name Active Cavity Radiometer (ACR). The temperatures of their cavity sensors are servocontrolled, actively maintained at constant temperatures relative to their heat sinks by electrical heating. The solar irradiance is related to the International System of Units by measuring the difference in amount of electrical heating required with the cavity sensor alternately shaded and exposed to the sun. Studies conducted with the ACR are discussed. While variations of a few tenths of a percent in solar total irradiance lasting no more than a week or two have been detected, no long term trends were identified.

Willson, R. C.↗

NASA NTRS1981

Variations of solar irradiance

The active cavity radiometer experiment on the Solar Maximum Mission is providing sensitive measurements of time variations of the total solar irradiance with greater accuracy and precision than previously achieved. The mean 1 AU irradiance for the first 45 days' operation is 1368.64 W/sq m with an absolute uncertainty of less than + or 0.5%. Variations about this mean have been observed on time scales of hours to days with amplitudes up to + or - 0.04%, resolved with a statistical uncertainty as low as 0.001%. Variations within a single orbit with amplitudes as large as + or - 0.5% have been resolved with 0.005% or smaller statistical uncertainty. Although these variations do not display a systematic relationship to conventional solar activity indices over the period, correlative behavior cannot be ruled out on the basis of the present limited data set.

Willson, R. C.↗

NASA NTRS1981

Observations of solar irradiance variability

High-precision measurements of total solar irradiance, made by the active cavity radiometer irradiance monitor on the Solar Maximum Mission satellite, show the irradiance to have been variable throughout the first 153 days of observations. The corrected data resolve orbit-to-orbit variations with uncertainties as small as 0.01%. Irradiance fluctuations are typical of a band-limited noise spectrum with high-frequency cutoff near 0.15/day; their amplitudes about the mean value of 1368.31 watts per square meter approach plus or minus 0.05%. Two large decreases in irradiance of up to 0.2% lasting about one week are highly correlated with the development of sunspot groups. The magnitude and time scale of the irradiance variability suggest that considerable energy storage occurs within the convection zone in solar active regions.

Willson, R. C.↗

NASA NTRS1981

Solar Maximum Mission experiment - Initial observations by the active cavity radiometer

The Active Cavity Radiometer on board the SMM is providing high-quality measurements of the solar irradiance. After correction for the solar distance, the orbital displacement of the satellite, and the relativistic shift of irradiance due to the satellite motion, the observed standard deviation is in the range 10-15 parts per million in a 96-minute integration. Measurable solar variations occur on time scales of a few minutes to a few days. The total amplitude of the variations in the daily averages from February 16 to March 31, 1980, was 0.10% based upon 96-minute averages.

Willson, R. C.↗

NASA NTRS1981

Sunspots and solar variability

The analyses of Willson et al. (1981) and Hudson et al. (1981) are extended in correlating the total solar irradiance monitor (ACRIM) data with the routine synoptic sunspot data. At the simplest level, this correlation reveals clearly that the so-called 'missing flux' is truly missing in the sense that large young active regions do produce at net diminution of the irradiance when their spots cross the central meridian. It is pointed out that the irradiance deficit must of course be made up, either promptly or on intermediate time scales; this is because the surface effects cannot perturb the energy generation processes in the interior. In the approach taken here, simple models of the reemission are constructed, the total reemission is scaled to the estimated sunspot deficit, and an attempt is made to measure the parameters of the models by a statistical comparison with ACRIM data.

Hudson, H. S.↗

NASA NTRS1980

Active cavity radiometer type V

The paper deals with a new type of cavity sensor geometry used in the most recent of a series of solar flux pyrheliometers, known as the active cavity radiometer (ACR). The new Type V sensor design incorporates a modification of its predecessor, the ACR IV sensor. This modification decreases the uncertainty within which cavity absorptance can be predicted by more than a factor of 10.

Willson, R. C.↗

NASA NTRS1980

Direct measurement of solar luminosity variation

Two rocket flights of an absolute pyrheliometer, separated by 30 months, indicate an increase in solar luminosity (solar constant) of 0.4 percent. The significance of this result is considered in light of the instrument performance during the rocket flights and of pre- and postflight intercomparisons with independently maintained pyrheliometers. There is a high probability that the measured difference is real. Additional observations are required to determine whether the difference results from random fluctuations in solar luminosity, a nonrandom change of short duration, or a sustained change that has climatological significance.

Willson, R. C.↗

NASA NTRS1979

Active cavity radiometer type IV

A new cavity pyrheliometer, the active cavity radiometer type IV (ACR IV), has been developed for the measurement of total solar optical irradiance. Analysis predicts its ability to measure at the solar constant level with 0.1% uncertainty in SI units. In comparison tests ACR IVs have consistently demonstrated 0.3% higher results than the World Radiometric Reference scale. A prototype has been tested, and a flight instrument has been developed and flown in a sounding rocket experiment to determine the solar constant. ACR IV instrumentation is being developed for flight experiments on the Spacelab I and Solar Maximum missions to monitor the total solar output of optical radiation as part of a long-term program to detect variations of climatological significance.

Willson, R. C.↗

NASA NTRS1978

Accurate solar 'constant' determinations by cavity pyrheliometers

Total solar irradiance was observed outside the earth's atmosphere by three types of absolute cavity pyrheliometer in a June 1976 sounding-rocket experiment. The 1367-W/sq m average solar 'constant' result is uncertain by less than + or - 0.5%, the most accurate determination to date. Nearly simultaneous observations by the Nimbus 6 earth-radiation-budget total-irradiance detector of 1389-W/sq m exceeded the rocket result by 1.6%. These recent results are discussed in the context of a summary of solar-constant determinations made above the troposphere by cavity pyrheliometers.

Willson, R. C.↗

NASA NTRS1978

Status of knowledge of the extraterrestrial solar 'constant' and spectral distribution

Current knowledge of the total solar flux (the solar constant) and its spectral distribution at the top of the earth's atmosphere is reviewed. The development of pyrheliometers and radiation scales is traced, noting that active cavity pyrheliometers have been developed to reduce measurement uncertainty to within 0.1/%. The weighted mean solar constant from ground-based, aircraft, balloon, spacecraft and sounding rocket observations has been calculated to be 1369 W/sq m. Future observations are planned in order to reduce uncertainty to the 0.1% level required for climatological studies. A comparison of the measurement and models of solar spectral irradiance obtained by Thekaekara (1969, 1974), Arvesen (1969) and Labs and Neckel (1968, 1975) shows that the spectrum of Labs and Neckel is most accurate in the 0.4 to 1.25 micron range, that of Arvesen is best in the 0.3 to 0.4 abd 1.25 to 2.5 micron ranges and the model of Labs and Neckel is best at wavelengths greater than 2.5 microns.

Willson, R. C.↗

NASA NTRS1977

Rocket calibration of the Nimbus 6 solar constant measurements

A sounding rocket experiment was performed in June 1976, in which the solar constant was observed simultaneously outside the earth's atmosphere by three types of absolute cavity radiometers (the Primary Absolute Cavity Radiometer and two Active Cavity Radiometers, Type IV) and duplicates of Nimbus 6 ERB/ESP (Earth Radiation Budget/Eclectic Satellite Pyrheliometer) solar channels. The preliminary average solar constant result from the cavity radiometers is 1367 W/sq m with an uncertainty of less than plus or minus 0.5% in SI units. The duplicate ERB channel 3 on the rocket gave a value of 1389 W/sq m, which agreed exactly with the Nimbus 6 ERB channel 3 measurement made simultaneously with the rocket flight.

Duncan, C. H.↗

NASA NTRS1977

Active-cavity radiometer/pyroheliometer

Device, using specular black cavity heater, temperature sensors, and electronics and electrodeposited cavity/thermal impedance structures, can achieve + or - 0.1 percent long-term absolute uncertainty at solar constant level. Dual-cavity configuration helps decrease sensitivity for heat-sink temperature drift.

Willson, R. C.↗

NASA NTRS1977

Rocket calibration of the Nimbus 6 solar constant measurements

Total solar irradiance was observed simultaneously outside the earth's atmosphere by three types of absolute cavity radiometers and duplicates of four of the Nimbus 6 Earth Radiation Budget (ERB) solar channels in a June 1976 sounding rocket experiment. The preliminary average solar constant result from the cavity radiometers is 1367 Wm (-2) with an uncertainty of less than + or - 0.5% in S.I. units. The duplicate ERB channel 3 on the rocket gave a value of 1389 Wm (-2) which agreed exactly with the Nimbus 6 ERB channel 3 measurement made simultaneously with the rocket flight.

Duncan, C. H.↗

NASA NTRS1977

Current pyrheliometry for total solar irradiance observations

A Type IV active cavity radiometer (ACR IV) was developed at JPL in 1975-76 as part of the NASA Weather and Climate Program. It is capable of defining the absolute radiation scale with an uncertainty near 0.1% and a resolution of 0.02% at the nominal solar 'constant' level. The ACR IV is the first pyrheliometer capable of measurements at the 0.1% level, which is the threshold of solar 'constant' variability of significance for climatological modeling. A prototype has been tested, and a flight instrument was flown in a 1976 sounding rocket experiment which determined a solar 'constant' value of 1368 W/sq m. A three-detector version of the ACR IV is being developed to monitor the solar 'constant' during 1979-80 as part of NASA's Solar Maximum Mission. Another ACR IV will measure the solar 'constant' as part of the Spacelab I mission in 1980.

Willson, R. C.↗

NASA NTRS1975

Instrumentation for measurements of solar irradiance and atmospheric optical properties

The Active Cavity Radiometer (ACR), an accurate absolute pyrheliometer, has been developed for measurements of total solar irradiance. It has been used to discover a -2.2% error in the International Pyrheliometric Scale and to make measurements in balloon flight experiments yielding a solar constant value of 136.6 (+ or - 0.7) mW/sq cm. New ACRs are being developed to monitor the total output of solar optical radiation in balloon, satellite and space shuttle experiments with long-term absolute uncertainty of + or - 0.1% or less. In a separate program, instrumentation for the measurement of the scattering and extinction of solar radiation by the atmosphere is being constructed to provide data for modeling atmospheric aerosol content. The aerosol models will facilitate computation of radiative transfer effects yielding quantitative net fluxes useful in evaluating the climatological impact of aerosols.

Willson, R. C.↗

NASA NTRS1974

Determination of optical parameters of atmospheric particulates from ground-based polarimeter measurements

This paper describes the theoretical analysis that is required to infer, from polarimeter measurements of skylight, the size distribution, refractive index and abundance of particulates in the atmosphere. To illustrate the viability of the method, some data obtained at UCLA is analyzed and the atmospheric parameters are derived. The explicit demonstration of the redundancy in the description of aerosol distributions suggests that radiation field measurements will not uniquely determine the modal radius of the size distribution. In spite of this nonuniqueness information useful to heat budget calculations can be derived.

Kuriyan, J. G.↗